1) Field of the Invention
The present invention relates to an improvement in non-aqueous electrolyte secondary cells.
2) Description of the Related Art
Non-aqueous electrolyte secondary cells, for their high energy density and high capacity, are widely used as power sources for mobile phones. In recent years, there has been rapid enhancement of functionality of mobile information terminals such as mobile phones and laptop computers, resulting in a need for cells with higher capacity.
While lithium cobalt oxide has been conventionally used as a positive electrode active material for non-aqueous electrolyte secondary cells, use of lithium nickel composite oxide instead of lithium cobalt oxide provides higher discharge capacity than the case of lithium cobalt oxide. Thus, lithium nickel composite oxide is increasingly expected to be used as a positive electrode active material for non-aqueous electrolyte secondary cells.
Lithium nickel composite oxide, on the other hand, has the property of intensely discharging oxygen when charged to high temperatures, posing the problem of a lack of safety. Further, for lithium nickel composite oxide of high quality to be obtained, synthesis under Li-rich conditions is effective; however, the alkaline component tends to remain on the surface of the lithium nickel composite oxide during synthesis thereof under Li-rich conditions, thus posing the problem of degraded high-temperature preservation characteristic.
Examples of the prior arts related to non-aqueous electrolyte secondary cells include Japanese Patent Application Publication Nos. 2001-307730 (patent document 1), 2002-75368 (patent document 2), 2003-292308 (patent document 3), 2004-87299 (patent document 4), 2005-123107 (patent document 5), 2005-183384 (patent document 6), 2005-276475 (patent document 7), and 2006-134770 (patent document 8).
Patent document 1 discloses use of, as a positive electrode active material, a composite of a first lithium compound represented by LiMPO4 (M including Fe) and a second lithium compound having a potential rarer than that of the first lithium compound. This technique is claimed to realize a cell having excellent charge/discharge characteristics.
This technique, however, poses the problem of insufficient high-temperature preservation characteristic.
Patent document 2 discloses covering the surfaces of LiNi1-xMxO2 particles (M being at least one selected from Al, B, and Co) with LiFePO4 fine particles. This technique is claimed to realize a cell provided with both characteristics of LiNi1-xMxO2, which has high energy density, and LiFePO4, which has limited degradation of capacity during charging.
This technique, however, poses the problem of insufficient high-temperature preservation characteristic.
Patent document 3 discloses covering the surfaces of LiFePO4 particles with a conductive carbon material while restricting the average particle diameter of the LiFePO4-carbon composite to 0.5 μm or less. This technique is claimed to improve discharge capacity.
However, the LiFePO4 particles have low fillability which makes high density filling difficult, thus failing to improve discharge capacity sufficiently.
Patent document 4 discloses a positive electrode active material made of lithium nickel oxide particles, the surfaces of which are covered with LiFePO4 having an olivine crystal structure. This technique is claimed to realize a cell provided with advantages of the lithium nickel oxide and those of the olivine compound.
This technique, however, poses the problem of insufficient high-temperature preservation characteristic.
Patent document 5 discloses use of, as a positive electrode active material, a composite of olivine lithium phosphate represented by LiFePO4 and a carbon material. This technique is claimed to realize a cell capable of rapid charging and having high capacity.
However, the LiFePO4 particles have low fillability which makes high density filling difficult, thus failing to improve discharge capacity sufficiently.
Patent document 6 discloses use of, as a positive electrode active material, a mixture of lithium nickel composite oxide and LiFePO4. This technique is claimed to realize a cell excellent in safety during overcharge.
This technique, however, poses the problem of insufficient high-temperature preservation characteristic.
Patent document 7 discloses use of, as a positive electrode active material, lithium iron phosphorus composite oxide. This technique is claimed to realize a cell having high discharge capacity and excellent charge/discharge cycle characteristics.
However, the lithium iron phosphorus composite oxide particles have low fillability which makes high density filling difficult, thus failing to improve discharge capacity sufficiently.
Patent document 8 discloses use of a positive electrode of a multi-layer structure composed of a first active material layer of LiNiO2 and a second active material layer of LiFePO4, laminated with one another. This technique is claimed to realize a cell excellent in continuous charging characteristic and high-temperature preservation characteristic.
This technique, however, still poses the problem of insufficient high-temperature preservation characteristic.